In an illumination device for lateral illumination, which is provided at a distal-end portion of an endoscope, a substantially semi-cylindrical light guiding member including a curved emission surface disposed at an outer peripheral surface of an insertion portion, a diffusion surface disposed so as to face the emission surface and having a light diffusion effect, and a proximal-end surface to which illumination light is supplied from a light guide has been conventionally used (for example, refer to PTL 1). The illumination light entering the proximal-end surface of the light guiding member from the light guide is reflected, by the diffusion surface, outwardly of the insertion portion in a radial direction. At this time, the illumination light is diffused by the diffusion surface, whereby brightness unevenness and color unevenness included in the illumination light emitted from the light guide are made uniform.